It is known to use a hydraulic piston to close a clutch pack in a clutch for an automatic transmission. It is desirable to minimize the time required to close such a clutch by reducing the travel needed for the piston to engage and compress the clutch pack. The travel can be reduced by minimizing spacing between clutch plates in the clutch pack. However, minimizing the spacing increases the viscous drag between the clutch plates, which increases losses for the transmission and increases fuel consumption for a vehicle using the transmission. Also, the greater the travel required for the piston, the greater the amount of fluid needed to displace the piston. Increasing the amount of fluid needed increases the work required of a pump for the fluid, further increases losses in the transmission.
U.S. Pat. No. 6,162,147 describes a single piston for a clutch assembly with two pressure chambers for displacing the piston. One of the chambers is pressurized more quickly than the other chamber. U.S. Pat. No. 3,915,274 describes a dual piston arrangement for a clutch assembly with dual pressure chambers and a common fluid input line. The two chambers are independently connected to the input line. That is, there is no direct flow of fluid between the two chambers. An outer piston is initially displaced by an increase in pressure in one chamber to contact move the inner piston into contact with a clutch pack. The movement of the inner piston due to contact with the outer piston increases a volume of the other chamber and pressure is increased in the other chamber in a timed relation to the pressure increase in the outer chamber. As pressure in the other chamber increases, the inner piston displaces independently of the outer piston to close the clutch pack.